[01] Sanjeev Kumar Shukla, R&D Centre for Iron & Steel (RDCIS), Steel Athority of India Ltd. (SAIL), Ranchi, India. [02] Mohammad Serfraj Alam, R&D Centre for Iron & Steel (RDCIS), Steel Athority of India Ltd. (SAIL), Ranchi, India. [03] Kartik Nageswaran, R&D Centre for Iron & Steel (RDCIS), Steel Athority of India Ltd. (SAIL), Ranchi, India. [04] Vinod Kumar, R&D Centre for Iron & Steel (RDCIS), Steel Athority of India Ltd. (SAIL), Ranchi, India. [05] Santosh Kumar, R&D Centre for Iron & Steel (RDCIS), Steel Athority of India Ltd. (SAIL), Ranchi, India.

To examine the influence of isothermal annealing treatment both below and above Ms temperature on the transformation kinetics and microstructure evolution in a medium carbon nano-structured bainitic steel, dilatometry studies were conducted in a Thermo-mechanical simulator (Gleeble-3500 C) in a steel of compostion C: 0.49%, Mn: 1.46%, S: 0.019%, P: 0.003%, Si: 1.78%, Cr: 1.12% & Mo: 0.17% having Ms temperature ~270°C. The heat treatment cycle comprised of heating the samples at 5°C/s to austenitizing temperature of 850°C, soaking at 850°C for 180 seconds, cooling at 20°C/s upto the isothermal annealing temperature varying from 180°C to 360°C, holding at these temperatures for 8 hrs. followed by natural cooling upto room temperature. Microstructure at and above Ms temperature comprised lower bainite (accompanied by carbide precipitation) alongwith retained austenite and martensite-austenite (MA) blocks, while below Ms temperature, besides tempered martensite, features of typical lower bainite were observed alongwith carbides aligned in ferrite platelets. Phase transformation kinetics was analysed through dilatometry plots. It has been found that formation of prior athermal martensite causes an acceleration at the start of the subsequent isothermal transformations below Ms temperature compared to those above Ms temperature, where prior martensite does not form, which is reflected in high incubation time for bainitic transformation in such cases.

Nano-structured Bainitic Steel, Phase Transformation Kinetics, Thermo-mechanical Simulation

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